The present invention relates to a method to determine the transmission quality of a signal transmitted by a base station and received by a mobile station in a cellular digital radio telephone network.
In cellular radio telephone networks a mobile station, e.g., a radio telephone to a car, is connected to the closest base station of the base station providing the best transmission quality. When the transmission quality drops below a predetermined acceptable level, the controlling exchange in the network will hand-off the mobile station to another base station providing a better transmission quality.
The various digital cellular systems (e.g. European GSM system and the US digital mobile telephone network) each have standardized criteria for determining the lowest different acceptable transmission level before a hand-off is triggered. The bit error rate (BER) of the signal received in the mobile station is the most common criteria. This BER value should then be transmitted to the respective base station in a channel quality message.
The bit error rate is conveniently determined (or measured) at the input to the decoder, i.e., the BER is determined before decoding, but after the digital signal has been received, demodulated, filtered and detected.
For example, the BER could be determined by an encoding-decoding method, where a received data frame is decoded and error corrected and then encoded and compared to the actually received frame. An estimate of the number of bit errors is provided by counting the number of unequal bits in the frames. This method is effective, but complicated to implement.